On March 1st, Wikipedia, the online interactive encyclopedia, hit the million-articles mark, with an entry on Jordanhill, a railway station in suburban Glasgow. Its author, Ewan MacDonald, posted a single sentence about the station at 11 P.M., local time; over the next twenty-four hours, the entry was edited more than four hundred times, by dozens of people. (Jordanhill happens to be the “1029th...